This invention relates to the field of magneto optic (MO) recording and, more particularly, to methods for producing aluminum nitride layers for use in magneto optic recording elements.
As used herein, "magneto optic recording element" refers to a multilayer structure used in the magneto optic write/read process for selectively modifying the characteristic of an incident polarized light beam in accordance with a magnetization pattern formed in a magneto optic recording layer which forms part of such structure. Typically, such recording element includes a substrate, a magneto optic recording layer (e.g. amorphous alloys of rare earth and transition metals) and one or more corrosion preventing, anti reflection layers of a dielectric or semi conductive material. A so called "bi-layer" recording element comprises a magneto optic layer having a single anti-reflection layer disposed on one surface thereof, the opposite surface of the magneto optic layer being contiguous with the supporting substrate (which is not counted in determining the number of layers). A "tri-layer" structure would include a second protective layer disposed between the substrate and the MO layer.
A primary function of the anti-reflection protective layer in an MO recording element is to enhance the magneto-optic rotation angle .THETA..sub.K caused by the well-known Kerr effect (in reflective systems) and Faraday effect (in transmission systems). As is well known, some of the more desirable MO materials exhibit a .THETA..sub.K of well less than 0.5.degree., and, without enhancement, the signal-to-noise ratio of the read-out signal, is marginal, at best. Another equally important function of the antireflection layer is to prevent corrosion of the contiguous MO layer. It is well known that some of the most promising MO layers (e.g. the aforementioned rare earth transition metal (RE-TM) alloys) are environmentally unstable. That is, they exihibit little resistance to corrosion and oxidation which, of course, dramatically alters their magneto-optic properties.
In a 1984 article Published in the Japanese Journal of Applied Electronics, Vol. 8, No. 2, pp. 93-96, entitled "Study on High Reliability of a Magneto-optic Medium with a Multilayer Structure," K. Ohta et al disclose a tri layer recording element in which a rare earth transition metal (RE TM) thin film is sandwiched between a pair of corrosion-preventing layers made of crystalline aluminum nitride (AlN). These layers are supported by a glass substrate, and the exposed AlN layer is overcoated with a reflective layer (e.g. copper). As a result of this multilayer arrangement, the magneto optic-rotation angle .THETA..sub.K, increased to 1.2 degrees, and the coercive force of the MO layer was relatively stable over time. While this enhanced value of .THETA..sub.K produces a relatively high figure of merit, .THETA..sub.K .sqroot.R, (where R is the power reflectivity of the recording element), the manufacture of such an element is relatively time consuming and, hence, costly. Moreover, the environmental stability of such elements is still far less than desirable.